Resonant circuits are often used for impedance matching in high frequency switching applications such as radio frequency (RF) switches. An example of one such RF switch is illustrated in prior art FIG. 1. RF switch 100 includes antenna port 120, transmitter port 110, and receiver port 130. In order to deliver a signal to be transmitted from transmitter port 110 to antenna port 120, a DC voltage is applied using voltage 198. The voltage is applied across bias resistor 195, through RF choke 190, through pin diode 150, on through quarter wavelength transmission line 170, and finally through pin diode 160 to ground. The DC current is prevented from entering ports 110 and 130 by blocking capacitors 180. When current flows through pin diode 160, one side of quarter wavelength transmission line 170 is effectively shorted to ground. Due to the properties of the quarter wavelength transmission line 170, the other side of quarter wavelength transmission line 170, which is not shorted to ground, is seen as an open circuit to a signal coming from transmitter port 110. In effect, the signal being transmitted from transmitter port 110 to antenna port 120 sees only the path between the two ports 110 and 120, and does not see the path to receiver port 130.
Conversely, when a signal received at antenna port 120 is to be delivered to a receiver connected to receiver port 130, voltage source 198 is turned off to stop the current flow through pin diodes 150 and 160. When no current flow through pin diode 150, pin diode 150 effectively acts as an open circuit, and so the signal received at antenna port 120 travels only along the path to receiver port 130. Note also that when no current flows through pin diode 160, which is coupled to one end of transmission line 170, the properties of quarter wavelength transmission line 170 are such that a signal having the proper frequency can travel through quarter wavelength transmission line 170 unhindered, as long as the impedance seen by the signal at port 130 matches the impedance seen by a signal of the same frequency at antenna port 120.
Matching networks 140 are used to try and match the impedance seen by a signal at antenna port 120 and the impedance seen by a signal at receive port 130. Under ideal conditions, matching networks 140 would be used to provide optimum power transfer from antenna port 120 to receiver port 130. In reality, however, matching networks 140 do not provide an exact impedance match between antenna ports 120 and receiver ports 130, and so there is signal loss across quarter wavelength transmission line 170, and maximum power is not transferred.
The imperfections in matching networks 140 arise in part due to unavoidable variations of component values used in constructing matching networks 140. In addition, when matching networks 140 are being designed initially, there comes a point where some particular component values for matching networks 140 must be decided upon and fixed. Variations in the values of other components, including pin diode 150 and pin diode 160, effectively change the resonant frequency of matching networks 140 so that the impedance match between antenna port 120 and receiver port 130 is somewhat degraded.
Another characteristic of prior art RF switch 100, one that makes RF switch 100 less desirable for use in mobile applications, is a large current draw. A property of pin diodes 150 and 160, and indeed pin diodes in general, is that the diodes' resistance and capacitance decrease as more current flows through them. Since it is desirable for pin diode 150 and pin diode 160 to have low resistance and capacitance values when turned on, it follows that a large amount of current must flow through pin diodes 150 and 160 to minimize signal losses due to high resistance, and to minimize impedance mismatches due to large capacitive values.
As should be apparent from the above discussion, currently available high frequency switches, and the matching networks used therein, could be improved. What is needed is a way to allow switching of high frequency signals between multiple ports that uses less current than the amount of current required by conventional pin diode switches such as prior art RF switch 100, and/or a way to provide more accurate impedance matching between different ports of a high frequency switch.